Heat pumps are often employed to provide heating or cooling to a target space or zone, often the indoor area of a residential or commercial building. The most common type of heat pump is the air-source heat pump, which transfers and amplifies heat between a target space and the air in another space, often an ambient environment. For heating, ventilation, and air conditioning/refrigeration (HVAC/R) applications, heat pumps often utilize the vapor-compression refrigeration cycle, in which a circulating refrigerant is used as the medium which absorbs heat from one space and subsequently rejects the heat elsewhere.
In a single-stage vapor-compression heat pump system, the refrigerant flows through an evaporator which absorbs heat and produces a vapor and then to a compressor that provides the necessary pressure increase before entering a condenser to reject the heat. The refrigeration is then expanded to a low pressure using an expansion device such as a thermal expansion device (TXV) before returning to the evaporator. Fans or blowers are also often used to transfer the heating or cooling effect to the target space or ambient environment. Single-stage vapor-compression systems are not practical for cold-climate heating applications due to the low suction pressure of the refrigerant at low ambient temperature and the difficulty of efficiently operating compressors at high compression ratios and compressing refrigerants with large specific volume.
One known cold-climate heating solution is a multi-stage cascade heat pump system, in which multiple separate vapor-compression cycles are coupled to each other with the evaporator of the higher-stage cycle removing the heat of the condensing refrigerant of the immediately lower stage cycle. Each cycle in a multi-stage cascade system usually uses a different refrigerant suitable for that temperature, with the refrigerant selected to be best suited for its operating conditions. Multi-stage cascade heat pump systems have the advantages of a lower evaporating temperature, smaller compression ratio and higher compressor volumetric efficiency when compared with single-stage systems.
Two-stage cascade systems have been used in HVAC/R for many decades. However, these traditional two-stage cascade systems suffer from inherent inefficiencies that result from the overlap of the condensing temperature in the lower stage and the evaporating temperature in the upper stage. This shortcoming can be partially avoided by introduction of a liquid-vapor heat exchanger and desuperheaters. However, there exists a continuing need to develop cascade heat pump systems with increased energy efficiency.